Field of the Disclosure
The present disclosure relates to a data clipping method for driving a display device and a display device using the same.
Discussion of the Related Art
Data clipping can be used to boost the luminance of lower grayscales to improve displayed electronic images as an alternative to increasing light output from an electronic display device. A data clipping method is a method in which highest grayscale pixel data is clipped from a histogram of an input image and multiplied by a gain to modulate the pixel data. A data clipping method will be described with reference to FIGS. 1A to 3B.
For an original test image of FIGS. 1A and 1B, data can be clipped as shown in FIGS. 2A, 2B, 3A, and 3B. The original image of FIG. 1B is divided into four regions: red (R), green (G), blue (B), and white (W) regions. The grayscale at the outermost edge of each of the red (R), green (G), blue (B), and white (W) regions is 0. The grayscale gradually increases toward the center of the region, and the pixel data at the center of each region has the highest grayscale, 255.
The graphs in FIGS. 1A, 2A and 3A plot the input grayscale along the X-axis and the output grayscale along the Y-axis.
The data clipping method of FIG. 2A is an example of hard-clipping pixel data including multiplying the original pixel data by a gain of 1.5. In the example of FIG. 2A, data from grayscales 171 to 255 is clipped and saturated at the data of the highest grayscale. In this case, data at high grayscales of 171 to 255 are saturated at data equal to that of grayscale 255 of the original test image. In this condition, an observer will experience a severe degradation in picture quality because this method effectively reduces the number of grayscales in the image and loses detail at the highest grayscales. In the data clipping method of FIG. 2A, the red (R), green (G), and blue (B) grayscales are saturated, whereas the white grayscale is not saturated, as illustrated in FIG. 2B.
The white grayscale is not saturated because, in an LCD, a portion of white luminance is represented by mixing light having R+G+B wavelengths generated by RGB sub-pixels and a portion of white luminance is also represented by light generated by a white (W) sub-pixel. Thus, in an LCD, because white light is represented by driving all the R+G+B+W sub-pixels, white luminance corresponding to grayscale 255 may be obtained even when a grayscale of W sub-pixel data is lower than 171. In case of an OLED Display, white luminance may be presented by driving only a W sub-pixel.
White data is obtained from RGB data received as an input image using a known white gain calculation algorithm. A spectrum exchanging method is known as one method using the algorithm. In the spectrum exchanging method, in order to reduce an amount of light having RGB wavelengths of RGB sub-pixels by an amount of light having RGB wavelengths generated by only a W sub-pixel, RGB data written in RGB sub-pixels is subtracted, and W data equivalent to the subtracted data value is generated to increase luminance of the W sub-pixel to compensate for the lowered white luminance of the RGB sub-pixels. In the case of an LCD, a portion of target white luminance is represented by mixing light generated by the RGB sub-pixels, and the other luminance is represented by luminance of the W sub-pixel. Thus, in the above example, even though the grayscale of the W data is not 171, white luminance corresponding to grayscale 255 may be obtained, and thus, grayscale of W data is not saturated.
FIGS. 3A and 3B show an example of soft clipping in which the gain is 1.5 before a point of inflection and less than 1 for grayscales above the point of inflection. It is difficult to achieve more than a certain level of grayscale representation in the soft clipping method due to a point of inflection caused when pixel data is multiplied by a gain.
FIG. 4 is a flowchart showing a control procedure for a data clipping method according to the related art.
Referring to FIGS. 4-6, in the data clipping method of the related art, the histogram of an input image is created, and the maximum frame value, frame_max, is calculated in consideration of data clipping. In this data clipping method, the number of pixels at each grayscale is accumulated starting from the highest grayscale successively downward in the histogram, and the count is continued until the total accumulated count value exceeds a predetermined threshold. The last grayscale just before the total count value exceeds the threshold is defined as the maximum frame value, frame_max, (S101 and S102). Pixel data at grayscales higher than the maximum frame value is clipped.
Once the maximum frame value, frame_max, is defined, the frame gain, frame_gain, is calculated. The frame gain frame_gain is calculated by dividing the highest grayscale, i.e., 255, by the maximum frame value, frame_max (S103). Pixel data can be clipped as shown in FIGS. 2A and 3A by modulating pixel data by multiplying the input pixel data by the frame gain (S104 and S105). The pixel data clipped at grayscales above the maximum frame value exceeds the highest grayscale value when multiplied by a gain, and is therefore substituted by the highest grayscale, i.e., 255. The data clipping improves the luminance of pixels at grayscales below the maximum frame value. The output of a backlight used to illuminate a liquid crystal display panel can be reduced by the amount according to an increase in the luminance of pixels in a liquid crystal display device, thereby reducing power consumption of the device.
Although the data clipping method can be used to increase the luminance of pixels at lower grayscales, the pixel luminance may not be increased depending on the image. FIG. 5 is an example of a histogram of a dark image. FIG. 6 is an example of a histogram of a bright image. In FIGS. 5 and 6, the x axis represents grayscales as the bins of the histogram, and the y axis represents the cumulative number of pixels for each bin (grayscale) of the histogram.
For the dark image of FIG. 5, the luminance of pixels can be increased by the amount of data clipping at grayscales above the maximum frame value frame_max. On the other hand, for the bright image of FIG. 6, the cumulative number of pixels for high grayscales is large, and therefore a higher grayscale is defined as the maximum frame value. Thus, the grayscales of pixels are saturated at high grayscales when pixel data is modulated with the frame gain. To avoid high-grayscale saturation, the bright image has little increase in luminance.